Ion cyclotron resonance spectrometer and method

ABSTRACT

An ion cyclotron resonance mass spectrometer having a source region and an analyzer region operable in both the standard drift mode and in a pulsed mode which includes ion trapping. Appropriate configurations of applied electrostatic fields permit trapping of ions in the source region of the spectrometer for relatively long periods after which detection is effected by drifting the ions from the source through the analyzer region where their power absorption is measured. The spectrometer may also be operated in the normal mode, thus allowing for the full range of conventional ion cyclotron resonance measurements with the additional capability of examining the variation of ion abundance with time.

United States Patent 1191 Beauchamp 1 Nov. 25, 1975 1 ION CYCLOTRON RESONANCE SPECTROMETER AND METHOD [76] Inventor: Jesse L. Beauchamp, 1780 San [44] Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. 8 455,520.

Related U.S. Application Data [63] Continuation of Ser. No. 298,341, Oct. 17 I972,

abandoned.

{52] US. Cl 250/291; 250/290 [51] Int. Cl. B01D 59/44 [58] Field of Search 250/281. 282, 290. 291, 250/292; 313/62 [56] References Cited UNITED STATES PATENTS 3,446,957 5/1969 Gielow et al 250/292 3,475,605 10/1969 Llewellyn 250/290 3,497,688 2/1970 Brown et a1. 250/290 3.502867 3/1970 Beauchamp 250/290 3.511.986 5/1970 Llewellyn 250/419 DS 3.742.212 6/1973 Mclver 250/419 DS OTHER PUBLICATIONS "An lon Ejection Tech. for the Study of lonMol.

Reac. with Ion Cyclotron Resonance Spectroscopy." Beauchamp et al.. Rev. Sci. Inst. Jan. 1969.

Trapped [on Anal. Cell for l.C.R. Spectroscopy," Mclver Rev. Sci. Inst, Apr. 1970.

Primary Examiner-James W. Lawrence Assistant Examiner-B. C. Anderson Attorney, Agent. or FirmFlehr, Hohback. Test. Albritton & Herbert (57] ABSTRACT An ion cyclotron resonance mass spectrometer having a source region and an analyzer region operable in both the standard drift mode and in a pulsed mode which includes ion trapping. Appropriate configurations of applied electrostatic fields permit trapping of ions in the source region of the spectrometer for relatively long periods after which detection is effected by drifting the ions from the source through the analyzer region where their power absorption is measured. The spectrometer may also be operated in the normal mode, thus allowing for the full range of conventional ion cyclotron resonance measurements with the additional capability of examining the variation of ion abundance with time.

8 Claims, 7 Drawing Figures US. Patent Nov. 25, 1975 Sheet 1 0f 3 3,922,543

US. Patent Nov. 25, 1975 Sheet 3 of3 3,922,543

TIME (m sec.)

cH cLcH TIME (m SEC.)

ION CYCLOTRON RESONANCE SPECTROMETER AND METHOD CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of application Ser. No. 298,341, filed Oct. 17, 1972, entitled Ion Cyclotron Resonance Spectrometer and Method, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to spectroscopy and more praticularly to an improvement in ion cyclotron resonance mass spectroscopy.

Ion cyclotron resonance mass spectrometers are well known and operate on the principle that a charged particle or ion moving in a uniform magnetic field absorbs energy continuously from an electric field which alternates at the characteristic frequency of motion of the ion in the uniform magnetic field directed at a right angle to the magnetic field. The characteristic frequency of the ion is a function of only the charge to mass ratio of the ion and the magnetic field intensity.

The omegatron mass spectrometer operates on this principle. In an omegatron, the mass is determined by knowledge of the intensity of the magnetic field and the frequency of oscillation of electronic circuitry which provides the alternating electric field. Additional electronic circuitry is employed to measure the number of ions which are absorbing energy at a given frequency. Spurious ions are often measured. This and other characteristics of the omegatron limit the obtainable resolution and senstivity and hence the range of applications of the omegatron.

Another mass spectrometer which operates on the cyclotron resonance principle is the instrument disclosed in US. Pat. No. 3,390,265 by Peter M. Llewellyn issued June 25, 1968. In this mass spectrometer an indication of the number of ions is obtained by measuring the amount of energy which the resonant ions absorb from the electronic circuitry providing the alternating electric field. The regions of ion formation and ion measurement are separate. The mass spectrometer does not have the undesirable features of the omegatron. It provides a means for determining many important characteristics of interacting ion and neutral species which are of fundamental importance to the study of chemical reactions.

An improvement in the ion cyclotron resonance mass spectrometry technique employing resonance energy absorption for determining the kinetics of chemical reactions is a pulsed ion cyclotron mass spectrometer. In this spectrometer, ions are formed during a short time interval. The ions are trapped for a relatively long period of time and then detected. Time sequencing results in simplified analysis of the observed mass spectrometer signals and chemical reaction kinetics are more easily elucidated. A disadvantage of this apparatus is that important features of the non-pulsed type of ion cyclotron resonance mass spectrometer are absent and the applications are, therefore, limited.

OBJECTS AND SUMMARY OF THE INVENTION It is a general object of the present invention to provide a versatile ion cyclotron resonance spectrometer and method.

An object of this invention is the provision of a method and apparatus for ion cyclotron resonance 2 spectroscopy which apparatus can be operated to combine the pulsed ion cyclotron resonance technique and the normal drift motion technique.

Another object of the invention is provision of an ion cyclotron resonance spectrometer in which primary ions are formed within a short time interval in a source region by electron impact ionization, trapped in the source region for a relatively long period of time and then permitted to drift through an analyzing region where they are detected.

A further object of this invention is the provision of an ion cyclotron resonance spectrometer and method of spectrometry in which time sequenced pulses allow ions to be formed during a controllable short period of time, reacted with other species for a controllable longer period of time, and the result is then detected.

The above and other objects of the invention are achieved by an ion cyclotron resonance spectrometer including a source region in which ions are formed by ionization, means for applying magnetic and electrostatic fields to trap the ions for a predetermined period of time in the source region. an analyzing region, means for releasing and causing the trapped ions to drift through the analyzing region, and means for detecting the ions in said analyzing region.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a perspective view showing an ion cyclotron resonance spectrometer cell in accordance with the invention.

FIG. 2 is a longitudinal sectional elevational view schematically showing the ion cyclotron spectrometer of FIG. 1 with voltages applied for trapping ions.

FIG. 3 is another sectional elevational view showing the ion cyclotron spectrometer cell of FIG. 1 with voltages applied for releasing and drifting the ions.

FIG. 4 is a schmetic block diagram of the ion cyclotron resonance spectrometer of the present invention.

FIG. 5 is a set of waveforms for use in explaining operation of the spectrometer shown in FIGS. 1-4.

FIG. 6 shows a typical trace of variation of ion intensity with time for ions observed in methyl chloride at 13.0 eV and 2.l l0" Torr.

FIG. 7 is a plot of log as a function of time for the three ions of the methyl chloride system.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown an ion cyclotron resonance spectrometer for exciting and detecting ions. The spectrometer includes three regions: a source region 11, an analyzing l2, and a collector region 13. The cell is immersed in a uniform uni-directional magnetic field which may be formed by permanent magnets or by electromagnets. One pole of a magnet is shown at 14, FIG. 4, with the parallel fields H extending through the cell in a direction perpendicular to the side plates. The magnetic field may be monitored by suitable monitoring means and controlled so that it is stable. The field strength may be in the order of 300 30.000 Gauss.

The spectrometer includes a pair of spaced trapping plates 17 and 18 which form the side walls of the spec: trometer cell and extend the length of the source and analyzing regions 11 and 12. Spaced plates 21 and 22 define the source region 11 and spaced plates 23 and 24 define the analyzing region 12. In accordance with the present invention, there is provided an end plate 26 which, together with the other plates, defines a source 3 trapping region. The interconnected plates 27 provide a trapping-field-free region in which the ions drift par' allel to the magnetic field and are collected by the plates.

In the ion source trapping region, a beam of electrons 31 is fed from a filamentary heater 32 through aligned apertures 33 formed in the side plates 17 and I8 and is collected by a collector 34. The beam travels through a pair of open mesh grids 36, 37 which are preferably in the order of 90 percent open and disposed between the filament and trapping plate 17 and between the trapping plate 18 and the collector 34, respectively. These grids, operated at the potential of the adjacent trapping plate, shield the trapping region of the cell from the bias voltages applied to the filament and collector and have been found to greatly reduce ion losses during the trapping period. The electron beam 31 serves to ionize gases inside the source region 11 by collision.

Referring particularly to FIG. 2, the voltages applied to the spaced plates 21, 22 and 23, 24 are shown for operation in a trapping mode for positive ions. Thus, zero voltages are applied to the spaced plates 21, 22 of the source region and to the end plate 26, while negative voltages are applied to the spaced plates 23, 24 in the analyzing region. The application of these voltages creates a potential configurations in the source region very much like that in a trapped ion analyzer cell. The space potential everywhere within the source region is positive with respect to the surrounding electrode and adjacent resonance region. This constrains the ions to move on equipotentials of the drift field which close on themselves within the source region of the cell. For trapping negative ions the polarity of the voltages shown applied to the trapping and analyzer drift plates are reversed.

In a typical experiment ions are formed by pulsing the electron source to form an electron pulse of predetermined energy and duration. Ions are produced within the cell by electron impact with the gas molecules. The ions are trapped by the combined electrostatic and magnetic fields.

The ions formed by the electron impact and trapped within the cell are constrained by the uni-directional magnetic field 11 to circular orbits in a plane normal to the direction of the magnetic field. The angular or cyclotron frequency of this motion is w q/mH where q/m is the charge to mass ratio, of the ion and H is the magnetic field strength. This equation is for cyclotron frequency in the absence of trapping electic fields.

It will be understood that the presence of the static trapping fields affects the motion of the ions and the cyclotron frequency. In any event, the ions orbit as indicated generally at 30, FIG. 2, within the source region.

In accordance with the present invention, ion detection is affected by switching the voltages on the upper, lower and end plates to the voltage shown in FIG. 3 for positive ions. The spectrometer then operates in the normal drift mode of operation and the ions drift out of the source region through the analyzer region of the cell where they are observed with a marginal oscillator detector. The change of voltage and action of the marginal oscillator fields causes the ions to drift generally in an increasing spiral 35, FIG. 3.

Referring more particularly to FIG. 4, a circuit for operating the described spectrometer is illustrated. The spectrometer cell is enclosed in an enclosed in an envelope 38 which is evacuated by pump 39. Sample gases are provided through conduit 40. The timing is controlled by a ramp generator 41 which cyclically forms a sawtooth ramp voltage which is applied to pulse generators 42 and 43. Pulse generator 42 is set to generate an output pulse when the voltage ramp reaches a predetermined level. The output pulse from the pulse generator 42 is applied to a gate 43 which serves to gate the power supply 44 to apply a voltage between the filament 32 and the side plate 17 whereby electrons are accelerated toward and through the aperatures 33 and through the source region to be collected by collector 34. Pulse generator 42 may include controls for varying the pulse width whereby the duration of the pulse of electrons may be varied and the power supply 44 may include means for varying the voltage whereby the velocity of the electrons can be varied to thereby control both the duration and energy of the electron pulse which ionizes the gases by impact within the source region 11. Alternately, the gate 43 can supply a bias to the grid 36 located between the filament and the trapping plate to turn the electron beam on in conjunction with the output of pulse generator 42.

The pulse generator 45 responds to the output of comparator 46 to form output pulses of predetermined duration. The comparator forms an output when the inputs from the ramp generators 41 and 47 are in coincedence. By using two ramp generators and a comparator it is possible to scan the delay between formation of the ionizing pulse and formation of the scanning pulse by pulse generator 45. The duration of the output pulse may be controlled.

A power supply 48 is shown connected through switching circuit 49 to the upper and lower plates 21 and 22 of the source region. In the quiescent state the switch 49 connects the voltages 0, 0 to the upper and lower plates as shown in FIG. 2. Likewise, a power supply 50 provides its output to a switching circuit 51 which, in turn, is connected to the upper and lower plates 23 and 24. In the quiescent state the switch 51 applies the voltages V, V which correspond to the voltages in FIG. 2. After the ionization and a predetermined time delay, the pulse generator 45 generates an output voltage pulse which switches the switches 49 and 51 to apply the voltages +V, V and +V, V to the upper and lower plates 21, 22 and 23, 24 as shown in FIG. 3. The pulse generator generates a pulse having a predetermined adjustable duration during which voltages are applied and the ions drift are shown at 37, FIG. 3, and are detected by a marginal oscillator, to the presently described. After the predetermined time, the switches 49 and 51 are switched to the trapping potentials.

Referring more particularly to FIG. 5, the pulse 52 indicates the pulse applied to the gate 43 which, in turn, projects the electron beam pluse having a duration and amplitude dependent upon the setting of the power supply and pulse generator. The pulse 54 shows the pulse generated by the pulse generator 45 which occurs a predetermined adjustable time delay 55 after the ionizing pulse during which period of time the ions trapped in the source region react.

As is well known, the detection is performed by a marginal oscillator. Marginal oscillator 56 coupled to the lower plate 24 of the analyzer region. The output of the marginal oscillator is applied to a boxcar integrator 57 which serves to integrate the marginal oscillator output during the duration of pulse 52 as illustrated at 58. The action of the boxcar integrator is to continuously analyze and normalize the output of the marginal oscillator to provide to the recorder 61 a signal which records the detected ions as a trace 62.

The instantaneous power absorption of ions at resonance is given by A(t) N(O)q E,-,|t/4m (1) where N(O) is the number of ions with mass to charge ratio m/q, E,, is the radio frequency electric field strength and t is the time the ions have been in the analyzer region. The drift velocity of ions in the resonance region is determined by the static electric field strength E and the magnetic field strength H according the relatio r i V cEXFi/H (2) Thus, knowing the drift velocity from Equation (2) and the length l of the resonance region, the drift time through the resonance region is given by 1' lH/cE (a) when H and E are perpendicular fields. Hence, the power absorption increases linearly with time, rising to a maximum at t=1', beyond which it falls to zero as the ions leave the resonance region. The boxcar detector utilized in the present experiments integrates the transient power absorption, giving the measured signal intensity I M tq rml (4) At a fixed observing frequency, higher mass ions come into resonance at proportionately higher magnetic field strength. Thus, it follows that the integrated power absorption will be directly proportional to ion mass. Hence, to obtain true relative signal intensities, the detector output must be divided by ion mass.

The usefulness and accuracy of the method has been demonstrated by the satisfactory reproduction of rate constants in methyl chloride. A typical trace of intensity vs time for the positive-ions observed in methyl chloride at l3.0 eV and 2.l l0" Torr is shown in FIG. 6. The reaction sequence occurring in this system is Ch,ClH*+CI-I,Cl BCI-LCICHJH-ICI. (6) A solutuon to the kinetic equations for a simple primary (P), secondary (S), tertiary (T) ion system such as this yields the abundances of the various ions as T= k k-,P(O)/k, k, (le"" /k e""":l/k,), 9 where P(O) represents the initial concentration of primary ions. A plot of log (relative ion abundance) vs. time for each of the ions in methyl chloride is shown in FIG. 7. The negative slope in FIG. 7 for the disappearance of CI-I,CI+ gives k =l.2Xl0'cm='. molecule sec".

Referring again to FIG. 4, an additional oscillator may be connected to the plate 22 of the source region. The oscillator 63 may then be used in double ion resonance experiments.

More particularly, the apparatus of the present invention can be operated in the normal ion cyclotron resonance mode by switching the switches 47 and 49 to apply the right-hand voltages and opening the gate 43 whereby a continuous stream of electrons are projected through the source region. The ions then drift through the analyzing region and provide an output in a conventional manner. Ion cyclotron double resonance experiments can also be performed with the apparatus by msking use of the oscillator 63.

Thus, it is seen that there has been provided a versatile trapped ion cell for ion cyclotron resonance spectroscopy. The cell may be used for conventional ion drift experiments and double resonance experiments,

6 and can in addition be used to perform trapped ion experiments.

I claim:

I. In an ion cyclotron resonance spectrometer, means forming an ion source region, means for ionizing gaseous substances within said ion source region to form a plurality of different ion species, an analyzing region, means for directing a magnetic field through said source and analyzing regions, means for applying static electric fields to said source region and analyzing region which together create a potential configuration with equi-potential lines which close themselves within the source region and which together with said magnetic field serves to constrain the different ion species to move in said source region on said equi-potential lines to remain in the source region for the same predetermined time, means for thereafter applying electric fields to said source and analyzing regions which together create a potential configuration which releases said different ion species after said predetermined time and causes them to drift from said source region through said analyzing region, and means for detecting the different ion species as they drift through the analyzing region.

2. An ion cyclotron resonance spectrometer as in claim I in which said source region and analyzing region are defined by spaced side trapping plates, upper and lower source plates at one end of said trapping plates to define the source region, upper and lower analyzer plates at the other end of said trapping plates to define the analyzing region and an additional plate disposed opposite the end of said source region cooperating with the other plates to set up the equipotential constraining fields.

3. An ion cyclotron resonance spectrometer as in claim 1 in which said means for detecting ions as they drift through said analyzing region includes a marginal oscillator.

4. An ion cyclotron resonance spectrometer as in claim 2 in which positive ions are trapped in said source region by applying zero voltage to said trapping plate and source plates and a negative voltage to said analyzer plates.

5. An ion cyclotron resonance spectrometer as in claim 4 wherein the ions trapped in said source region are released and allowed to drift through said analyzer region by applying a positive voltage to the upper source and analyzer plates and a negative voltage to said lower source and analyzer plate and said trapping plate.

6. An ion cyclotron resonance spectrometer as in claim 1 wherein said ionizing means comprises a pulse of electrons of predetermined energy and duration.

7. An ion cyclotron resonance spectrometer as in claim 6 in which the analyzing means comprises a marginal oscillator and a boxcar integrator which is on during the drift time of the ions through the analyzer region.

8. The method of measuring ion intensity of constituents in a reaction comprises forming different ion species by impacting a pulse of electrons on a gas to be analyzed in an ion source region, constraining said different ion species so that they move only in said source region for the same predetermined period of time and then allowing the ions to drift from the source region through an analyzing region of an ion cyclotron resonance spectrometer to measure the signal intensity of each of said ion species.

i l l 

1. In an ion cyclotron resonance spectrometer, means forming an ion source region, means for ionizing gaseous substances within said ion source region to form a plurality of different ion species, an analyzing region, means for directing a magnetic field through said source and analyzing regions, means for applying static electric fields to said source region and analyzing region which together create a potential configuration with equi-potential lines which close themselves within the source region and which together with said magnetic field serves to constrain the different ion species to move in said source region on said equi-potential lines to remain in the source region for the same predetermined time, means for thereafter applying electric fields to said source and analyzing regions which together create a potential configuration which releases said different ion species after said predetermined time and causes them to drift from said source region through said analyzing region, and means for detecting the different ion species as they drift through the analyzing region.
 2. An ion cyclotron resonance spectrometer as in claim 1 in which said source region and analyzing region are defined by spaced side trapping plates, upper and lower source plates at one end of said trapping plates to define the source region, upper and lower analyzer plates at the other end of said trapping plates to define the analyzing region and an additional plate disposed opposite the end of said source region cooperating with the other plates to set up the equipotential constraining fields.
 3. An ion cyclotron resonance spectrometer as in claim 1 in which said means for detecting ions as they drift through said analyzing region includes a marginal oscillator.
 4. An ion cyclotron resonance spectrometer as in claim 2 in which positive ions are trapped in said source region by applying zero voltage to said trapping plate and source plates and a negative voltage to said analyzer plates.
 5. An ion cyclotron resonance spectrometer as in claim 4 wherein the ions trapped in said source region are released and allowed to drift through said analyzer region by applying a positive voltage to the upper source and analyzer plates and a negative voltage to said lower source and analyzer plate and said trapping plate.
 6. An ion cyclotron resonance spectrometer as in claim 1 wherein said ionizing means comprises a pulse of electrons of predetermined energy and duration.
 7. An ion cyclotron resonance spectrometer as in claim 6 in which the analyzing means comprises a marginal oscillator and a boxcar integrator which is on during the drift time of the ions through the analyzer region.
 8. The method of measuring ion intensity of constituents in a reaction comprises forming different ion species by impacting a pulse of electrons on a gas to be analyzed in an ion source region, constraining said different ion species so that they move only in said source region for the same predetermined period of time and then allowing the ions to drift from the source region through an analyzing region of an ion cyclotron resonance spectrometer to measure the signal intensity of each of said ion species. 